Hi Patrick,
Welcome to Auctiva! The quantity in inventory is actually supposed to be reduced by the quantity you list to eBay when you post new items. Then, if the listing were to come to an end before all of the items had sold, your inventory quantity would be replenished by the quantity of the item that did not sell.
The reason that the inventory quantity is reduced when items are listed (as opposed to when they sell) is to keep sellers from posting more items to eBay than they have available to sell. Many sellers have large enough quantities of each items that it is not reasonable to post all of their inventory to eBay at the same time.
Auctiva's inventory functionality can be used to warn you when your inventory levels reach a certain point. These options can be specified in the "Notifications" section that you are prompted to fill out when adding each inventory item using the "Add Inventory Item" link under the "Inventory" tab within your account.
If you have any additional questions regarding your Auctiva account, please feel free to contact our customer support team using the web form on the following page: http://www.auctiva.com/help/requesthelp.aspx
-Mike